This invention relates to the field of medical devices, and more particularly relates to battery-powered implantable medical devices.
It has been proposed in the prior art that it would be desirable to provide an implantable medical device having a power supply that is separate from the remaining components, so that when the power supply becomes depleted, the battery can be replaced without replacing the device itself. A device with a detachable and replaceable battery would also offer the advantage that different sized batteries can be utilized in accordance with patients' individual needs. One arrangement for providing a removable battery in an implantable medical device was proposed in U.S. Pat. No. 4,010,760 to Kraska et al., entitled "Coupling Assembly for Implantable Electromedical Devices", which patent is hereby incorporated by reference in its entirety.
When an implantable device having its power supply and electronics in a single housing is produced, steps can be taken during manufacture to ensure that the amount of water vapor in the housing is very low, on the order of ten parts per million (PPM) or so. The hermetic seals provided by the welded seams in typical implantable device housings are extremely effective in preventing body fluid from entering the housing. Thus, current leakage within the device due to fluid leakage is not generally a problem.
However, hermetically sealed connectors for use external to such device housings are not available. Typical implantable connector systems employing sealing rings or other similar sealing mechanisms are not hermetically sealed. In the absence of a hermetic seal, fluid will inevitably enter the connector. Since the terminals connecting a replaceable battery to a medical device will be at different potentials, if any significant fluid ingress occurs, current leakage will occur across the fluid, resulting in a potentially significant drain on the power supply, and creating the potential for corrosion. The Kraska et al reference proposes a solution to the problem of corrosion due to the expected fluid ingress by appropriate selection of connector metals. However, the Kraska et al reference does not address the problem of the resulting leakage current.
Connector systems designed for use in connecting implantable leads to pulse generators, for example, typically display leakage resistances in the range of 50 k-ohms. These connector systems employ sealing rings (similar to the Kraska et al reference) to separate each connector surface and to seal entry to the connector assembly. If a connector assembly of the type designed for coupling a lead to an implantable device were used for the alternate purpose of conveying battery voltage to an implanted device a 50-k.OMEGA. leakage resistance of a lead connector assembly would allow leakage currents in the range of 60 microamps or more, depending on battery voltage. In the context of an implantable pacemaker, this current leakage may well exceed the current drain of the pacemaker by a factor of 10 or more, which would be clearly unacceptable.